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On the weekend I did an overdue drivetrain change. Everything went fine and runs rather smoothly, but I had to adjust the front derailleur a lot. It seems like the new cranks are about 5 mm further away from the axle than the old ones.

I changed from Shimano FC TY301 to Shimano FC-M361.

Is the described discrepancy a likely case, or did I simply not push the cranks far enough onto the axle?

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  • A quick internet search shows a 50mm chainline spec for both cranks. Can you take a photo? – Michael Mar 23 '20 at 10:13
  • Sure, but could you specify what exactly you'd need to see in the photo @Michael? One crank, the axle? – Erik Mar 23 '20 at 10:52
  • Does your BB allow the crankshaft centering to be adjusted? – Daniel R Hicks Mar 23 '20 at 11:48
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I think what is going on here is that you were running a 47.5mm chainline (standard for MTB triples, see here) with the FY301 crank, but the M361 crank gets you a chainline of 50mm.

When switching out three-piece cranks you have to pay attention to the chainline and you also have to look at cartridge bottom bracket axle lengths and potentially replace the BB along with the crank. Different model cranks require different BB axle lengths as the axle interfaces of different models are not in the same position long the axle axis relative to the chainrings.

Shimano helpfully provides extensive specification documentation for their products but figuring out the proper BB for a crank is somewhat complicated. We can find the crankset ('chainwheel' in Shimano parlance) specs here.

Do a text search on that page for 'TY301' and 'M361' and you should find the entries we want. Note the TY301 has a chainline spec of 47.5mm (normally, there is a 50mm option too) and the M361 has 50mm. The TY301 should be paired with a BB-UN100 or BB-UN26 bottom bracket, axle length code D-NL (for 47.5mm chainline). The M361 should be paired with a BB-UN26 bottom bracket, axle length code LL123.

If we look at the bottom bracket specs here (search on the page for 'UN26') we see that the LL123 code means 123mm, D-NL means 122mm (if I read the specs correctly). 1mm difference isn't enough to worry about.

I think if you measure the chain line with the M361 crank on (distance from the center of your frame seat-tube to the center of the middle ring) it will be about 50mm. If your measured chainline is about 55mm, you must have had the 50mm option on the TY301 crank, so you definitely need to replace the BB with the correct one for the M361.

If your original chainline was 47.5mm, you could replace the BB with one with a 118mm axle to drop your chainline back, or you could just run 50mm. If you derailleur can reach the big ring OK I'd be tempted to leave it as it is. If you plan to replace the BB make sure there is >5mm of clearance between the rings and the chainstay - presumably there should be, unless you got larger rings with the new crank.

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  • I am not completely satisfied yet, but yes, the front derailleur works with the new chainline spec. Thanks for the extensive input! – Erik Mar 23 '20 at 11:53

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